1. Field of the Invention
The present invention relates to a method of time measurement in a communications system, and to a communications system and a receiving apparatus for use in such system. It has particular application to cordless telephone and selective call., such as digital radiopaging, systems.
For convenience of description the invention will be described in the context of a digital radiopaging system.
2. Description of the Related Art
U.S. Pat. No. 4,845,491, issued Jul. 4, 1989 discloses a digital pager which is operable in accordance with the CCIR Radiopaging Code No. 1 standard otherwise known as POCSAG. In order for a user to be able to review chronologically messages stored in a memory, each message is stamped with the date and time of receipt. Date and time message signals are periodically transmitted by a paging network controller (PNC). However with respect to the time message signals, delays of up to 15 minutes may occur before they actually are transmitted. In order to correct for such errors in the pager clock, the next following time message signal as transmitted includes an indication of the error in the previously transmitted time message signal, that is delay in its transmission. Thus the pager clock can be updated on the basis of the time indicated in the previous time message signal (t.sub.n-1), a correction C.sub.n representative of the difference between the time indicated in the previous time message signal and the time of its actual reception, and the time difference, as measured by a real time clock, between the times of receipt of the previous (T.sub.n-1) and of the current (T.sub.n) time message signals. The cited prior art patent gives the following formula for determining the updated time T.sub.n(new), viz EQU T.sub.n(new) =t.sub.(n-1) +C.sub.n +(T.sub.n -T.sub.(n-1)).
Whilst this known technique for determining real time is accurate, it nevertheless requires two message transmissions from the paging network controller and a pager receiver capable of comparing the time indicated in each time message against a reference time standard.